Wave transmission system



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R E T R A C F E WAVE TRANSMISSION SYSTEM Filed April 20, 1934 INVENTOR BY fin ATTORNEY Patented Sept. 17, 1935 UNITED STATES PATENT OFFICE WAVE TRANSMISSION SYSTEM Application April 20, 1934, Serial No. 721,493

8 Claims.

This invention relates to wave transmission systems and with particularity to amplifier systems employing electron-discharge tubes or the like.

An object of the invention is to provide an improved method and means for biassing the control electrodes of a plural-stage amplifier.

Another object of the invention is to provide means to compensatively bias the control-grids of a plural-stage amplifier.

A feature of the invention relates to a pluralstage amplifier employing grid-control tubes, wherein the tendency towards bias change in one stage due to signal voltage changes, is compensated for under control of the plate current of another stage.

Another feature relates to a push-pull amplifier having means to compensate for tendency of the control-grid bias to change with changing plate current.

A further feature relates to a plural-stage vacuum tube amplifier having a common biassing resistor so connected that the signal variations on the control-grid of one stage tending to produce changes in grid bias are compensated under control of the plate current of a preceding stage.

A further feature relates to a plural-stage amplifier comprising a push-pull stage and a driver stage arranged so that the push-pull stage is over-biassed negatively while the driver stage is under-biassed negatively, together with means for maintaining the bias on the push-pull stage substantially constant under control of the driver stage when signals are being transmitted. One example of an over-biassed amplifier of this type is a Class A-prime amplifier.

A still further feature relates to the novel organization, arrangement and relative connection of parts which go to make up an improved amplifier system.

Other features and advantages not specifically enumerated will be apaprent after a consideration of the following detailed descriptions and the appended claims.

While the invention will be described herein as embodied in one particular type of wave transmission system, it will be understood that this is done merely for illustrative and explanatory purposes and that the drawing shows in schematic form only those portions of a typical form of amplifier system necessary for an understanding of the invention.

Referring to the drawing the numeral l represents diagrammatically, any well-known form of wave repeater device such for example as a grid-controlled vacuum tube. Numerals 2 and 3 represent similar grid-controlled devices such as thermionic vacum .tube repeaters or the like. The device I may comprise an evacuated vessel containing an electron-emitting cathode 4, a control grid 5, and an anode 6 such as ordinarily employed in radio amplifier tubes. While the drawing shows cathode 4 of the so-called indirectly heated type adapted to be rendered thermionically active by a heated filament 1, it will 5 be understood that any other type of electronemitting cathode may be employed. Similarly the tubes 2 and 3 respectively contain electronemitting cathodes 8, 9, control grids IO, M, anodes !2, l3 and heater filaments l4, I5. 10

Numeral It represents any well-known source of signal wave variations to be repeated and/or amplified, for example the detected output of a radio receiver or the like. The signal variations are impressed across the cathode and control- 15 grid of tube I through any suitable coupling means. For example these variations are shown in the drawing as being impressed across a coupling resistor I! which in turn is coupled to grid 5 through a suitable coupling condenser I8. In 20 accordance with the usual practice a leak resistance E9 is provided, which has one end connected directly to grid 5 and the other end connected through condenser 20 to cathode 4, condenser 29 being of sufficient capacity to pass all the re- 25 quired frequencies to be repeated and/or amplified. The output circuit of tube 1 is traceable from the steady high positive potential terminal 2!, primary winding 22 of an interstage coupling transformer, anode 6, cathode 4, conductor 23, 30 resistor 25, to the negative terminal 26 of the steady source of potential, this latter terminal being preferably grounded as shown. Preferably resistor 25 is shunted .by a condenser 21 to divert the alternating components of the plate current 35 variations from said resistor. The normal plate current flowing through the resistor 25, when no signals are impressed on grid 5, causes an IR drop in said resistor, a suitable portion of which may be used to bias the grid 5 negatively with 40 respect to cathode 4. For this purpose the lower end of resistance I9 is connected through a resistance 28 to a predetermined point on resistor 25 as shown. Because of the IR drop in the upper section 29 of the biassing resistor, the pre- 45 selected negative bias is applied to grid 5. Preferably the connections are such that grid 5 is under-biassed slightly negatively, so that when the negative bias is increased in the manner described hereinbelow, a corresponding predeter- 0 mined reduction in the total IR drop through resistance 25 is effected.

The secondary winding 30 of the interstage coupling transformer is connected at opposite ends to control-grids l0, ll of tubes 2, 3 which, as shown, are connected for push-pull operation. For this purpose the electrical mid-point of winding 30 is grounded and cathodes 8, 9 are connected to ground and the negative terminal 26 through biassing resistor 25. The output circuit for this push-pull stage is traceable from the steady high positive potential terminal 3|, to the balanced primary winding 32, thence in parallel to anodes l2, l3, cathodes 8, 9, and through resistor 25 to negative terminal 26. The secondary winding 33 of the output transformer is connected to any suitable load as indicated schematically by numeral 34. The normal anode current of tubes 2 and 3 fiows through resistor 25 and thus biases grids l0 and II to a predetermined negative potential with respect to their associated cathodes 8, 9. Preferably the connections and values are such that when no signals are impressed, the grids l 0 and I l are over-biassed negatively, such as is ordinarily done in connection with Class A-prime amplifiers.

From the foregoing description it will be seen that the IR drop through resistor 25 is a function not only of the steady plate current of tube 2 but also of the steady plate current of tube I. From the foregoing description it will be seen that the IR drop through resistor 25 is a function not only of the d. 0. component of the plate current of tube 2 but also of the d. 0. component of the plate current of tube I. Since the tubes 2, 3, are normally over-biased negatively, relatively low normal steady space current flows therethrough when no signals are being trans mitted. When however signals are impressed, the positive half waves of these signals cause marked increase in the normal space current, as distinguished from the amplitude of the alternating signal component. In ordinary biassing systems therefore the bias will tend to vary, particularly on the positive half waves of the impressed signals. By suitably over-biassing grids ID and l! negatively, and under-biassing grid 5 negatively, the changes in d. c. component of the plate current of tube 2 tending to change the bias on grids is and II may be counterbalanced by the opposite changes in plate current of tube I so that there is achieved a substantially steady bias on grids i9 and i 5 over a relatively wide variation of signal potentials impressed on said grids.

Various changes and modifications may be made herein without departing from the spirit and scope of the invention.

What I claim is:

l. A wave transmission system including a pair of grid-controlled electron-discharge devices, means for negatively biassing the control-grids of said devices under control of the d. c. component of the anode currents thereof, the first device being normally under-biassed and the second device being normally over-biassed; the last mentioned means being in common to the gridcathode circuits of both devices, and circuit connections for causing the d. 0. component of the plate current of one device to oppose a tendency to grid-bias change in the other device.

2. A wave transmission system including a pair of grid-controlled electron-discharge devices, a biassing resistor in common to the grid-cathode circuits of each device, and circuit connections including said resistor and the anode cathode circuit of each device for normally under-biassing the first device and over-biassing the second device and for maintaining the overall drop through said resistor substantially constant while signals are being transmitted.

3 A wave transmission system including a pair of grid-controlled electron-discharge devices connected in cascade, a single resistor connected between the cathode of each tube and the negative terminal of the anode supply, and a connection from the control-grid of one of the tubes to a point intermediate the ends of said resistor, said resistor serving normally to under-bias the first device and to over-bias the second device.

4. An amplifier including a pair of grid-controlled electron-discharge devices connected in cascade, a single resistor connected in the output circuit of one device and having a portion connected in the output circuit of the other device, the cathodes of both devices being connected to the same end of said resistor, the grid of one device being connected to the opposite end of said resistor, and the grid of the other device being connected to a point intermediate the ends of said resistor to cause the plate current of said other device to oppose any tendency to change in bias on the grid of said one of said devices, said resistor serving normally to under-bias the first device and to over-bias the second device.

5. In combination, a first amplifier tube, a second amplifier tube, a resistor having one end connected to the cathodes of both tubes and the other end connected to the negative terminal of the anode supply, circuit connections for causing the steady anode current of both tubes to fiow through said resistor, a connection for causing the total IR drop through said resistor to be applied to the grid of the second tube, and a connection for applying only a portion of the said IR drop to the grid of the first tube, said IR drops being proportioned normally to under-bias the grid of the first tube negatively and normally to overbias the grid of the second tube negatively.

6. In combination, a first amplifier tube, a second amplifier tube, means for normally underbiassing the control-grid of the first tube negatively with respect to its cathode and for normally over-biassing the grid of the sec-ond tube negatively with respect to its cathode, said means including a single resistor common to the output and input circuits of the second tube and having only a portion common to the output and input circuits of the first tube, said common portion being proportioned with respect to the total resistance of the resistor so that the first tube is under-biassed and the second tube is over-biassed and tendencies of the grid-bias on the second tube to change with signal voltage are counter-balanced under control of the plate current of the first tube.

7. In combination, a pair of amplifier tubes, a single resistor through which the d. 0. component of the anode current of each tube passes to bias the grid of said tubes negatively to their cathodes, and circuit connections for applying the plate current of one of said tubes to counterbalance a tendency of the control-grid bias of the other tube to change with applied signal voltage, said resistor serving normally to underbias the first tube and to over-bias the second tube.

8. In combination, a driver tube stage having its control-grid normally under-biassed negatively, a push-pull tube stage having the controlgrids normally over-biassed negatively, the biassing means for said tubes including a single resistor having only a portion common to the input circuits of the driver stage and of the pushpull stage, the total resistor being connected between the cathodes of the push-pull stage and the negative terminal of the anode supply.

EMMETT FINLEY CARTER. 

